This invention relates to superconducting states and processes and products useful in achieving superior superconducting properties and results.
Of the two commercially available and useful bulk superconductors, NbTi and Nb.sub.3 Sn, NbTi is used in more applications because it is more ductile while Nb3Sn has superior superconducting properties but, in comparison to NbTi, its useful forms have generally had less desirable mechanical properties.
The absence of good ductility characteristics of Nb.sub.3 Sn type superconductor structures has been substantially overcome by the so called "bronze" process, whereby the niobium and Sn are processed as separate metallurgical phases in a composite. A reaction heat treatment is applied to the finished wire. During this treatment, tin from the copper-tin bronze alloy reacts with the niobium to form the A-15 crystal structure of Nb.sub.3 Sn at temperatures below 930.degree. C. Copper is required as another element to stabilize or catalyze the formation of Nb.sub.3 Sn over more tin-rich niobium phases.
Typical structures and methods for their preparation are shown in U.S. Pat. Nos. 4,262,412 and 4,414,428 to McDonald, the descriptions of which are incorporated herein by reference in their entirety.
So called modified jelly roll (MJR) as described in the foregoing patents, is incorporated herein by reference and is used in the processes for preparing the products of this invention.
While variations of many of the bronze processes exist including those which comprise alloys with various other elements such as Ga, Ti, Mg.sup.1,2, and the like, and the niobium filament can be further alloyed with small amounts of Ti, Ta, Zr, Hf, Sn, Fe, and V, the overall current density in such structures is affected, i.e., diluted, by the amount of tin-containing bronze that is required to convert substantially all the niobium to Nb.sub.3 Sn.
Increased ductility in wire drawing processes is achieved by separating the tin and copper as distinct phases which can be heavily worked without the annealing normally necessary with bronze after working.
In addition to the improved ductility, the separation of tin and copper allows the volume fraction of niobium to be increased while maintaining an adequate volume of tin for complete reaction. Thus, the Nb content of the non-copper volume of the wire, and subsequently Nb.sub.3 Sn, is less diluted by the nondispersable metallic content.
There can be further improvements where, as in the above-described MJR process, the tin, copper and niobium materials required to form the Nb.sub.3 Sn are surrounded with a layer of barrier material which is selected from materials which effectively prevents diffusion of tin from the tin core into the copper matrix of the wire remotely located from the niobium and the subsequently formed Nb.sub.3 Sn filaments. Wire thus formed contains both copper and non-copper areas, after FNT .sup.1 I. W. Wu, et al, "The Influence of Magnesium addition to the bronze on the critical current of bronze-processed multifilamentary Nb.sub.3 Sn", IEEE, Trans Mag-19, pp. 1437-1440, 1983. FNT .sup.2 K. Togano et al, "Effects of Magnesium addition to the Cu-Sn Matrix in the Composite-Processed Nb.sub.3 Sn Superconductor", Jour. of Less-Common Metals, 68, (1979), 15-22. reaction. Where the tin has reacted, substantially completely, both with the niobium and all the copper within or inside the barrier layer, the Nb.sub.3 Sn filaments are surrounded by bronze.
While magnesium has been used heretofore in the production of niobium-containing products by the bronze process, that use causes substantial unwanted mechanical and physical anomalies resulting in a loss of ductility and poor workability when fabrication is attempted as well as problems with the higher temperatures required for processing.
As previously described, the most commonly used Nb.sub.3 Sn wire product is made by the bronze process.
In many applications, magnetization, by definition, is a concern. A superconducting body has significant magnetization because of its ability to pin fluxoids (which is also why it can carry high currents). The magnetization at any field will be directly related to J.sub.c (the critical current density).
Magnetization (M) is also directly related to the superconducting area and hence the filament width.
.DELTA.M.alpha.J.sub.c d where d=filament diameter
While it would appear obvious that .DELTA.M can be reduced by decreasing d, the reality is different. As the diameter is decreased, so is the spacing between filaments. During the heat treatment, closely spaced filaments may "bridge" together creating an effective filament diameter which is much larger (20-100 times) than an individual filament. This undesirable result is compounded by the expansion of the filament on conversion from Nb to Nb.sub.3 Sn. From measurements of .DELTA.M and J.sub.c a d.sub.eff (effective filament diameter) can be calculated.
One object of the present invention is to increase and enhance the critical current of Nb.sub.3 Sn wire or rods while reducing the amount of bridging between filament elements.
Still another object is to control the growth of Nb.sub.3 Sn grain size and insure the homogenous unimpeded distribution and diffusion of small grains by employing the conditions and structure which provide a homogenous distribution of Mg along the length of wire.
Yet another object is to produce, in situ at the locus of the niobium filament, finer grain growth of smaller grains of Nb.sub.3 Sn (after reaction) and Mg.sub.2 Sn (before reaction) to provide a wire structure capable of being worked into filamentous wire or rods for superconducting products.
A further objective is to provide a method of fabricating a wire structure capable of achieving the superconducting state while exhibiting a Nb.sub.3 Sn current density greater than about 2600 Amps/mm.sup.2 at 10 Tesla 4.2.degree. K. and 10-13 ohmmeters.
A still further object of the present invention is to reduce the occurrence of bridging between filaments to thereby maintain as small an effective diameter of filament as possible after fabrication and reaction.
These and other objects are all accomplished while still retaining the fabricability of the metallic products such as wire and rods by the addition of small amounts of magnesium to the tin core of the tin core MJR.
We have unexpectedly found that magnesium content at the eutectic composition (.about.98% Sn, .about.2% Mg) is enough to produce a dramatic increase in current density (overall) of about 20-30% without further dilution of the Nb.sub.3 Sn current density, and produces a very fine 2(d) phase Mg.sub.2 SN and the desired homogeneous dispersion by the radial diffusion of the eutectic either inwardly or outwardly.